Will Bad Spark Plugs Throw A Code? [All FAQs]

Yes, a bad spark plug will throw a code. It can throw misfire codes on multiple cylinders, banks, and coils somewhere in the P0300 and up range.

Symptoms of a Bad Spark Plug

Examining your spark plug can provide information about what is going on in your combustion chamber.

A spark plug covered in black soot, for example, indicates a rich condition, whereas a white spark plug indicates a lean condition. 

It can also tell you whether your engine is too cold or too hot, so knowing how to read the condition of your spark plugs is critical.

Below are some symptoms that indicate a fouled spark plug.

6 Symptoms of a Bad Spark Plug Keep an eye on these Failing Signs when to replace it

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Lack of Power

When your spark plugs are worn or damaged, the first thing you’ll notice is a decrease in your vehicle’s performance.

It may not always turn on your check engine light or send you a code until the spark plug completely fails.

This symptom may result in:

  • Slow acceleration
  • Reduced gas mileage
  • Engine stalls

Engine Misfire

Engine misfires occur because one or more cylinders fail to produce power due to a severely worn plug that cannot ignite the air-fuel mixture.

You might also feel your car vibrate excessively or lose power when it misfires. 

When your car’s check engine light continues to flash, it indicates multiple misfires.

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Engine misfires may also occur from a clogged fuel injector or a faulty oxygen sensor.

Hard Starts

When your engine has a bad spark plug, it will result in hard starts. This problem is due to excessive carbon buildup in your plug. 

Carbon buildup shortens the life of your plug and can cause hard starts, decreased acceleration, engine misfires, and the appearance of the check engine light.

Engine stall

When your engine stalls, it is a sign that your throttle system is malfunctioning with hard starts. It states that your plugs are worn out and are not producing enough spark to start your engine. 

As a result, your engine will stall and refuse to start.

Rough Idling

Your engine will sound smooth and steady if your spark plugs are working properly. A bad spark plug, on the other hand, makes sputtering sounds. 

Additionally, you might also notice that the vehicle is trembling.

Unusual Fuel Consumption 

A bad spark plug may cause you to refuel more frequently or less frequently.


A contaminated fuel and oil is another reason your spark plug may not work properly.

These contaminated liquids can cause a carbon-fouled spark plug, resulting in incomplete combustion in the engine. 

The following are some factors for contaminating:

  • Clogged air filter
  • Excessive richness in the fuel/air mixture
  • Clogged fuel injectors
  • Idling your car for a long period
  • Low-speed driving

Symptoms Of  Bad Spark Plugs

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Frequently Asked Questions

Can bad spark plugs throw P0420?

A P0420 code can indicate a problem with the catalytic converter, which reduces its efficiency, or a problem with the oxygen sensors.

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So, yes, a bad plug can throw this code.

If this code appears, it is recommended that the plugs be checked.

Can bad spark plugs throw lean codes?

Yes. When the fuel doesn’t burn properly, some raw fuel is expelled through the exhaust.

An 02 sensor determines whether the engine is rich or lean by measuring the amount of oxygen in the exhaust. 

When this occurs, fuel is drawn from that bank of cylinders. You will eventually develop a lean code.

Can bad spark plugs throw P0171?

The most common cause of a P0171 (too lean, Bank 1) code is a vacuum leak from the MAF sensor or a malfunctioning MAF sensor. Usually, the opposite code could be caused by bad spark plugs (too rich, bank 1). 

So, no, it is unlikely.

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